1. Field of the Invention
The present invention relates to a hydraulic control apparatus for controlling a hydraulic cylinder for an implement selectively mounted on a work vehicle.
2. Description of the Related Art
An example of a work vehicle using the above-noted type of hydraulic control apparatus is a tractor type front loader. For instance, in the case of such front loader known from Japanese Patent Application “Kokai” No.: Hei. 9-316915, a boom to which a bucket is pivotally attached is mounted to a tractor frame. And, a boom cylinder for lifting up/down this boom and a bucket cylinder for pivoting the bucket are controlled by hydraulic control valves provided respectively therefor. In this case, the hydraulic control valve for the bucket cylinder is constructed as a three-position changeover control valve having a rollback position, a neutral position and a dump position. Further, at the dump position, there is formed a so-called “regenerative circuit” in which a return pressure oil discharged from the bucket cylinder is combined with feed pressure oil to the bucket cylinder. Hence, this dump position functions as a rapid dump position. In this rapid dump arrangement using a regenerative circuit, the bottom of the bucket cylinder is rapidly filled with pressure oil, resulting in a rapid pivotal movement of the bucket for facilitating removal of sticky or viscous soil and sand from the bucket. In this way, the three-position changeover control valve is convenient for controlling a bucket. However, when a different implement other than a bucket is attached, such rapid cylinder movement may deteriorate the control convenience, rather than improve it. However, if such other implement is driven not by a hydraulic cylinder, but by a hydraulic motor, there may occur critical failure of the hydraulic motor being disabled at all by the function of the regenerative circuit which renders the pressure at the IN port equal to that of the OUT port. Therefore, the three-position changeover control valve is not suitable as a changeover control valve in a case where a variety of implements can be selectively attached to the work vehicle.
In order to solve this problem, as shown in FIG. 7, for controlling a bucket cylinder 15, there has been proposed to employ a four-position changeover control valve 400 constructed by adding a rapid dump position RD supplemented with a regenerative circuit as a fourth position to the conventional three-position changeover control valve having a rollback position RB, a neutral position N and a dump position SD. With this four-position changeover control valve 400, however, when an operator manipulating a bucket for dumping its load simply operates this control valve from the neutral position to the dump position adjacent thereto, the bottom of the bucket cylinder 15 may not be filled with the pressure oil because the velocity of the oil is insufficient for the movement of the piston cylinder. Further, when the control valve is returned from the dump position to the rollback position after completion of the dump operation, the bucket will not be moved until the bottom of the bucket cylinder 15 is filled with the pressure oil. Therefore, in dumping load from the bucket, the operator needs to operate the control lever from the neutral position to the rapid dump position directly, passing the dump position. At the rapid dump position, the bottom of the bucket cylinder 15 is rapidly filled with the pressure oil, whereby sticky and viscous soil and sand or the like can be readily removed from the bucket. However, since the operator needs to operate the control lever from the neutral position to the rapid dump position directly, passing the dump position, the operator will suffer from inconvenience if he/she has to effect in repetition the above operation for operating the control lever from the neutral position to the rapid dump position directly, passing the dump position. Incidentally, in FIG. 7, numeral 300 denotes a four-position changeover valve for controlling a boom cylinder 14, having an UP position, a neutral position N, a down position D and a floating position F.